The rock avalanche that dammed the Laguna Baya wasformed in the last 40-50 years as the result of slope instability processes inthe Guanaco headquarters, an affluent of the Pehuenche and Grande rivers in thesouthern Mendoza Province. This ageis determined by comparing air photograms taken in the 60´s with recentlyreleased TM images. This rock avalanche deposit constitutes one of the firsthistorical occurrence in the Argentinian Andes.